The use of intact vascular beds to investigate in transcapillary exchange limits the amount of information that can be obtained. Elucidation of the serial barriers molecules must face in their transit from blood to lymph is at best difficult and at worst impossible. We have chosen to look in detail at the first such barrier, the vascular endothelial cell, and have maintained such cells from both bovine and human origin in monolayer culture. At both the light and electron microscopic level we will compare the morphology of these cultured cells with their in vivo counterparts. In addition, arterial versus venous differences will be examined. Using tracer molecules of known molecular size, we will investigate the permeability of such cells to large and small molecules and attempt via morphological tracers to define the intracellular pathways. Agents known to alter vascular permeability such as histamine and bradykinin will be used in order to detect possible differences between normal and stimulated cells. Since these cells secrete large amounts of prostaglandins in culture we will assess the role of prostaglandins in regulating vascular permeability.